Field of the Invention
The present invention relates to a current mode control type switching power supply device that can perform a step down operation for stepping down an input voltage.
Description of Related Art
Control methods of switching power supply devices can be roughly divided into a voltage mode control and a current mode control. In general, the current mode control is very effective in view of simplification of phase compensation, fast response, and reduction of the number of external components. An example of the current mode control type switching power supply device is shown in FIG. 14.
A switching power supply device 100 shown in FIG. 14 senses current flowing in an upper metal oxide semiconductor (MOS) transistor Q1 so as to perform the current mode control. In accordance with the current mode control, the upper MOS transistor Q1 and a lower MOS transistor Q2 are complementarily turned on and off, and an input voltage VIN is converted into a pulse-like switched voltage VSW by this switching operation. Then, the switched voltage VSW is smoothed by an inductor and an output capacitor and is converted into an output voltage VOUT lower than the input voltage VIN.
When the current flowing in the upper MOS transistor Q1 is sensed so as to perform the current mode control, a fed-back portion of the current corresponds to a difference between the input voltage and the switched voltage (VIN−VSW), and hence a current sensing circuit generates information of the sensed current with respect to the input voltage YIN. Accordingly, when the current information is transmitted to a slope circuit configured to generate a slope voltage VSLP with respect to an internal source voltage, there occurs a delay time D after the upper MOS transistor Q1 is turned on until the current information is transmitted to the slope voltage VSLP, as shown in FIG. 15.
In addition, because the fed-back portion of the current corresponds to the difference between the input voltage and the switched voltage (VIN−VSW), if noise is added to a leading edge or the like of the switched voltage VSW, the noise is transmitted as it is and is reflected on the slope voltage VSLP.
Further, when a pulse width of the switched voltage VSW is decreased, the above-mentioned delay time and noise become dominant, and there occurs a problem that current feedback cannot be performed.
Note that the current mode control type switching power supply device disclosed in JP-A-2010-220355 also performs the current mode control by sensing current flowing in an upper switching element similarly to the switching power supply device 100 shown in FIG. 14, and hence has the same problem.